1. Field of the Invention
The present invention relates to a spot welding apparatus for spot-welding a workpiece in the form of a plate assembly.
2. Description of the Related Art
Generally, a spot welding technique is widely used for joining stacked plates, such as steel plates. The spot welding technique involves using a pair of welding electrodes to clamp and apply a pressure to the stacked plates, and applying current between the welding electrodes for a predetermined time period.
Referring to FIG. 7A, in the case of spot-welding a workpiece 100 in the form of a plate assembly including three stacked plates, i.e., a thin plate 101 having a lower rigidity, and a first thick plate 102 and a second thick plate 103 having a higher rigidity than the thin plate 101, when the workpiece 100 is held and pressed between a movable electrode 111 and a fixed electrode 112, the thin plate 101 having a lower rigidity and the first thick plate 102 bend upward, causing gaps to form between the thin plate 101 and the first thick plate 102 as well as between the first thick plate 102 and the second thick plate 103. In this case, the contact area between the movable electrode 111 and the thin plate 101 is increased due to the bending of the thin plate 101, whereas the contact area of the joint section between the thin plate 101 and the first thick plate 102 and the contact area of the joint section between the first thick plate 102 and the second thick plate 103 are reduced due to the gaps. Therefore, the current density between the movable electrode 111 and the fixed electrode 112 at the second thick plate 103 side becomes higher than that at the thin plate 101 side. This results in a greater local calorific value between the first thick plate 102 and the second thick plate 103 than between the thin plate 101 and the first thick plate 102.
As a result, as shown in FIG. 7A, a nugget 105 is first formed at the joint section between the first thick plate 102 and the second thick plate 103. Then, as shown in FIG. 73, the nugget 105 gradually grows so that the thin plate 101 and the first thick plate 102 are ultimately welded to each other. However, because the amount of weld penetration between the thin plate 101 and the first thick plate 102 is small, the weld strength is poor, and the welding quality varies. This problem becomes prominent particularly when the thickness of the first thick plate 102 and the thickness of the second thick plate 103 are increased since the increased thicknesses make it difficult for the nugget 105 to reach the joint section between the first thick plate 102 and the thin plate 101.
Japanese Unexamined Patent Application Publication No. 2003-251469 discloses an example of a spot welding method as a countermeasure against this problem. Specifically, as shown in FIG. 8, when spot-welding the workpiece 100 formed of three stacked plates, i.e., the thin plate 101, the first thick plate 102, and the second thick plate 103, a pressure FU of a movable electrode 125 at the thin plate 101 side is set to be lower than a pressure FL of a fixed electrode 124 at the second thick plate 103 side. Thus, the contact resistance between the thin plate 101 and the first thick plate 102 is increased, whereas the contact resistance between the first thick plate 102 and the second thick plate 103 is reduced. Accordingly, when current is applied between the movable electrode 125 and the fixed electrode 124, the calorific value at the joint section between the thin plate 101 and the first thick plate 102 is increased, thereby increasing the weld strength between the thin plate 101 and the first thick plate 102.
FIG. 9 illustrates a spot welding apparatus used for implementing this method. Specifically, a spot welding apparatus 120 is attached to a wrist 116 of a welding robot 115. The welding robot 115 moves the spot welding apparatus 120 to each spot position of the workpiece 100 supported by a clamper 118 so as to spot-weld the workpiece 100.
The spot welding apparatus 120 includes a base 122 that is vertically movably supported by a linear guide 121. The linear guide 121 is fixed to a support bracket 117 attached to the wrist 116. A fixed arm 123 extending downward is provided on the base 122. The fixed electrode 124 is provided at a distal end of the fixed arm 123.
A pressure actuator 126 is attached to an upper end of the base 122 and is configured to move a rod 127 vertically. The movable electrode 125 is attached to a lower end of the rod 127 so as to face the fixed electrode 124. A servomotor 128 is attached to an upper end of the support bracket 117. The servomotor 128 is configured to move the base 122 vertically by means of a ball screw mechanism.
Based on teaching data stored in advance in a controller (not shown), the pressure FU of the movable electrode 125 located at the thin plate 101 side is set to be lower than the pressure FL of the fixed electrode 124 (FU<FL).
In order to set the pressure FU of the movable electrode 125 to be lower than the pressure FL of the fixed electrode 124 in this manner, the controller first causes the servomotor 128 to move the base 122 upward so as to bring the fixed electrode 124 into contact with a lower surface of the workpiece 100, and causes the pressure actuator 126 to move the movable electrode 125 downward so as to bring the movable electrode 125 into contact with an upper surface of the workpiece 100.
Subsequently, the base 122 is moved upward by the servomotor 128. When the base 122 is moved upward, the pressure FL of the fixed electrode 124 increases by an amount corresponding to the distance of the upward movement of the base 122, whereby the pressure FU of the movable electrode 125 becomes lower than the pressure FL of the fixed electrode 124 (FU<FL).
As a result, when current is applied between the movable electrode 125 and the fixed electrode 124, the current density at the joint section between the thin plate 101 and the first thick plate 102 becomes high, causing the calorific value to become relatively higher than the calorific value at the joint section between the first thick plate 102 and the second thick plate 103. Consequently, a good uniform nugget is formed that extends from the thin plate 101 to the second thick plate 103, thereby ensuring a high weld strength.
According to the above Japanese Unexamined Patent Application Publication No. 2003-251469, the fixed electrode 124 is brought into contact with the second thick plate 103 of the workpiece 100 held by the clamper 118, and the movable electrode 125 is brought into contact with the thin plate 101. Further, the pressure FU of the movable electrode 125 is set to be lower than the pressure FL of the fixed electrode 124 by moving the base 122 upward. Then, the current density between the thin plate 101 and the first thick plate 102 becomes relatively higher. Thus, a sufficient calorific value can be obtained at the joint section between the thin plate 101 and the first thick plate 103, thereby achieving an increased amount of weld penetration and increased weld strength.
However, when setting the pressure FU of the movable electrode 125 to be lower than the pressure FL of the fixed electrode 124 by moving the base 122 while the workpiece 100 is held by the clamper 118 and is held and pressed between the fixed electrode 124 and the movable electrode 125, a large load is placed on the clamper 118 holding the workpiece 100. On the other hand, if the position of the workpiece 100 held by the clamper 118 and the welding position of the workpiece 100 are markedly distant from each other, the workpiece 100 deforms and bends. This causes the pressure FL of the fixed electrode 124 and the pressure FU of the movable electrode 125 to vary, making it difficult to obtain stable contact resistance between the thin plate 101 and the first thick plate 102 and stable contact resistance between the first thick plate 102 and the second thick plate 103. This may result in variations in the current density at the joint sections, leading to reduced spot welding quality.